Two
spacewalking astronauts finally removed a faulty pump outside the International
Space Station Wednesday after overcoming a stubborn ammonia hose during their second
attempt in less than a week to repair the outpost's cooling system.

It
came down to brute strength in the end for NASA astronauts Douglas Wheelock and
Tracy Caldwell Dyson to remove the balky ammonia hose, which had stuck fast and
leaked toxic ammonia coolant during an earlier
spacewalk.

With
the hose out of the way, the spacewalkers were finally able to remove the
oven-sized cooling system pump after a slight delay attaching a handle-like
bar. The pump failed 11 days ago and knocked out half of the space station's
cooling system. [Graphic:
Space Station's Cooling System Problem Explained]

"This will allow us to get our station back," Wheelock
said. "Back in action."

Wheelock
had to shake a hose connector hard to pry it free from the broken pump that his
helmet camera showed him swaying from side-to-side. But ultimately he
disconnected the hose to the delight of the astronauts and Mission Control. He
saw only a few flakes of ammonia while detaching the pump's final fluid hose
Wednesday, far less than the snowstorm of frozen ammonia he reported seeing
during the Saturday spacewalk.

The
astronauts parked the broken pump at a storage point outside the space station
after removing it. NASA plans one more spacewalk,
currently scheduled for no earlier than Monday, to complete the ammonia pump
repair.

"We
still have a little ways to go, but today was a great day," Wheelock
radioed Mission Control after the spacewalk, which lasted seven hours and 26
minutes.

The
International
Space Station uses liquid ammonia to cool its onboard systems by
transporting waste heat to a network of radiators mounted to its main truss.
Spacewalking astronauts take care to avoid exposure to ammonia (which freezes
into flakes when it leaks) while working outside and have lengthy
decontamination protocols to clean their spacesuits when they do see leaks in
order to avoid bringing the toxic chemical inside.

The
faulty ammonia
pump failed July 31, forcing astronauts to turn off some experiments and
systems, as well as leave others without backups, in order to prevent the
station from overheating. An internal electrical short tripped a circuit
breaker to cause the malfunction, station managers have said.

Broken space pump removed

NASA station managers have said the ammonia pump failure is one of
the most challenging repairs for the International Space Station. Engineers
initially planned two spacewalks to replace the pump, but unexpected challenges
on Saturday forced the addition of a third spacewalk.

Part of the difficulty lies in the sheer bulk of the pumps. Each pump weighs 780
pounds (353 kg) and is 5 1/2 feet long (1.6 meters) by 4 feet wide (1.2
meters). They are about 3 feet (almost 1 meter) tall. So the spacewalkers had
to work together to move the bulky pump.

Caldwell
Dyson, who helped her electrician father work while growing up, ?disconnected
the pumps's five power and data cables. They gave her little trouble.

"My
dad would be proud," she added.

"I'm
just going to hold it as steady as I can," Wheelock told Caldwell Dyson as
he grasped the pump from the tip of the station's robotic arm. Station
astronaut Shannon Walker flew the arm for Wheelock from inside the station.

The
failed ammonia pump is located on the station's right side truss and will be
replaced with one of four spare pumps stored at the orbiting lab. The spare
pump that will replace it was arrived at the station in 2007.

But Wednesday's spacewalk appeared to go as planned, unlike the
first repair attempt. Wheelock
and Caldwell Dyson expected to have close two valves in hoses upstream of the
pump and vent any remaining ammonia coolant overboard before removing the stuck
ammonia hose, which was the last of four connected to the disabled pump. But
that wasn't needed, Mission Control said.

There
are two main cooling
system loops ? Loop A and Loop B. The failed pump is in Loop A, while the
other cooling loop remains operational. It was delivered to the space station
in 2002, but wasn't activated until 2006.

While
the space station is currently stable, the U.S. segment is running with just
half of its cooling system working until the new pump can be installed. If the
working cooling loop fails, astronauts would have only a short time to finish
the repairs, mission managers have said.

The
space station's Russian segment has its own independent cooling system, but
could only support the outpost's full six-person crew on its own for a few days
before station managers would have to consider last-minute repairs or
abandoning the station.

More
repairs ahead

Wheelock
and Caldwell Tyson didn't plan on completing their space station repairs during
Wednesday's spacewalk. That comes on Monday, when the astronauts will perform a
third spacewalk to retrieve a spare pump from a storage platform and install it
in the old pump's place, station officials have said.

Wednesday's
spacewalk began at 8:27 a.m. EDT (1227 GMT) and the astronauts were
consistently ahead of schedule.

"Sunlight
? sunlight coming out," Wheelock said as he looked out of the station's
open hatch just before stepping out to begin the spacewalk.

"Wow,
that's nice and bright," Caldwell Dyson responded.

"Beautiful,"
Wheelock said.

While
Wednesday's spacewalk was long, it fell short of the record-setting spacewalk
by Wheelock and Caldwell Dyson on Saturday. That spacewalk lasted eight hours
and three minutes, making it the longest spacewalk ever performed from the
space station.

Wednesday's
spacewalk was the fifth for Wheelock and the second for Caldwell Dyson. It was
the 149th dedicated to space station assembly and maintenance. Construction
began on the $100 billion space station in 1998.

Spare
parts are a key concern for the International Space Station, which is now slated
to keep flying through at least 2020. After next year, no NASA space shuttles
will be flying to the space station to deliver large spare parts or supplies.

NASA
plans to fly two more shuttle missions (in November and February 2011) before
retiring its orbiter fleet for good next year. NASA would need approval from
Congress and the White House to fly a potential third shuttle mission, which
would launch next summer if approved. Last week, the Senate passed a NASA
authorization bill that would approve that extra shuttle flight, though the
House vote on its version of the bill is still pending.